Tone and color reproduction have been studied in detail and more thorough discussions of the consequences of each can be found in numerous references such as "The Theory of the Photographic Process 4th Ed.", Chapter 19, edited by T. H. James and published by Macmillan Publishing Co., Inc. of New York, or "The Reproduction of Colour 4th Ed.", Chapter 13, by R. W. G. Hunt and published by Fountain Press, Tolworth, England.
The problems found in designing color papers with preferred color and tone reproduction have not gone unnoticed by the photographic industry. Indeed, there are numerous choices of color negative films and color negative papers presently available to match any variety of photographic needs. To date however, no combination of color negative film and paper has succeeded in producing a clearly preferred tone and color reproduction combination.
Generally, in the design of color negative printing papers, it is necessary to consider the range of exposure and contrast in the previously exposed color negative film. Since the purpose of the color negative paper is to receive color and tonal information previously recorded in the negative, the paper must be designed so that the resulting color print is pleasing to the eye both in color, contrast and tonal range.
Additionally, it is highly desirable that color negative printing papers have speed (sensitivity) and contrast characteristics that are invariant to exposure time. This feature permits their usage in a wide variety of applications, including high speed printers, easel printing and other electronic printing devices. Exposing devices for color papers may include light sources consisting of tungsten lamps, halogen lamps, lasers, light emitting photodiodes (LED's) or other light sources. To accommodate this variety of exposing devices, the emulsions used in the color negative papers are capable of recording the exposure between the exposure range of nanoseconds (1.times.10.sup.-9 seconds) to several minutes while maintaining printing speed and contrast.
Color negative films are designed to have inherently broad exposure ranges. This feature allows for over and under exposures which are common in amateur and professional film markets. Many commercially available color negative films have exposure ranges which allow the photographer to under expose the film up to 2 stops (0.6 log exposure), or to overexpose by as much as 4 stops (1.2 log exposure). This broad exposure latitude provides greater flexibility for amateur photographers and insures that the entire original scene lightness range is captured by the color negative film.
The contrast of the color negative material may, however vary between film types. For example, portrait photographers may prefer films of lower contrast to provide prints which appear less contrasty than a standard amateur color negative film. Amateur films are designed for a wider variety of applications. Higher contrast color negative films may be the choice of commercial photographers wishing to draw greater attention to their work by increasing color saturation and contrast.
Clearly, it is important for the negative film to maintain a constant contrast (or gamma) over this broad range of exposure. Failure to do so would cause contrast mismatches as a function of exposure when printing onto the color negative paper, resulting in incorrect color and poor tone reproduction.
The color negative paper must be adjusted for contrast so as to provide a preferred color print with the appropriate color negative. Historically this has been achieved by manufacturing a series of color negative films and papers each having a range of contrasts. For example, the Kodak Ektacolor 74.TM. and Kodak Ektacolor 78.TM. papers which differed primarily in contrast, were the choice of many professional and amateur photo finishers, for many years. These products were improved upon by Kodak Ektacolor Professional.TM. and the Kodak Ektacolor Plus.TM. color negative papers which also differed in contrast. More recently, new color papers have been introduced into the market widening the contrast range from low to medium to high. Products such as Kodak Ektacolor Portra.TM. (lowest contrast), Kodak Ektacolor Supra.TM. (medium contrast) and Kodak Ektacolor Ultra.TM. (highest contrast) utilize the rapid processing cycle known worldwide as the Kodak Ektacolor RA-4 Color Paper Process.TM., further increasing their utility and ease of use in the marketplace.
As the contrast of any color negative paper increases however, a problem occurs in the higher exposure areas, (darker area of the print) which lose detail and become `blocked in`. The paper appears to lose printing latitude resulting in the loss of shadow detail. Reducing the contrast of the paper to lighten the dark areas results in lower than desired contrast, desaturated colors and blacks (shadows) which are insufficiently black. This problem is especially troublesome when photographing weddings or brightly lit outdoor scenes with heavy shadows.
In wedding scenes it is especially difficult to print a white wedding dress with sufficient detail as to be pleasing and yet in the bridegrooms black tuxedo, keeping the folds in a sleeve from disappearing and becoming `blocked in`, for example. A similar problem exists in outdoor scenes where there exist a very wide range of scene luminance's. For example, on a sunny day with shadows from buildings or trees, it is especially difficult to print the directly lit area dark enough to have detail and keep the areas covered by shadow in shadow, light enough to see the detail in the shadow.
Custom printing using techniques such as masking, or dodging and burning are frequently used by professional print finishers to aid in printing scenes which contain large differences in scene illumination to reduce the difficulty. A fill-in flash is often used to illuminate dark shadow areas in the scene and reduce scene contrast.
Another factor to be considered when designing a color paper is the effect of the viewing conditions under which the print is to be viewed. Viewing flare results from the light reflected onto the print in the viewing room. The effect of adding reflectance to the color print is to reduce reflection density and hence contrast. Since the effect of viewing flare is logarithmic, areas in the print of higher density are affected more so than areas of lower density. Thus the result of viewing flare is to reduce upper scale density which reduces contrast and reduces detail in shadows.
The use of blended emulsions in a color photographic system is not new or novel. For example, to obtain wider exposure latitude in color negative film, Wyckoff in U.S. Pat. No. 3,849,138 describes combining emulsions of different sensitivity but equal slopes (contrast) in separate layers. Ohya, et al, in U.S. Pat. No. 5,039,601 describe a process for manufacturing silver halide grains of different sensitivity by growing together mixed silver halide seed grains where the seed grains themselves differ in inherent sensitivity. Patzold, et al, in U.S. Pat. No. 4,301,242 describes a process for preparing silver halide grains differing in sensitivity for color reversal papers. Emulsions used in these examples have different sensitivities but the same slope (contrast). Thus when combined, the exposure latitude is increased, but the contrast of the material remains unchanged. Asami, et al, in U.S. Pat. No. 4,745,047 describes the use of blended emulsions of different grain sizes in a color paper format to maintain gradation (contrast) in high density areas when benzyl alcohol is removed from the development process. Their use of blended emulsions is to maintain contrast, not increase contrast in high density areas when benzyl alcohol is removed from the color development process.
Buhr and Franchino U.S. application Ser. No. 950,871 filed Sep. 24, 1992 describes a system "for presenting a preferential tone mapping in a reproduction of an original scene, the system being of the type having means for capturing original scene parameters onto a medium from which a reproduction of the original scene is to be derived and means for creating a visual reproduction of the scene from the captured scene parameters." However, there is not disclosed therein a way to practice their invention for the conventional color negative positive film system for either a color negative film or color paper.
In Kuwashima U.S. Pat. No. 4,792,518, the recognition is made that in order to make an exact reproduction of a color transparency or reflection original, the characteristic S-shaped D vs. log-E curve should be avoided and a preferred curve having the shape of a "saw" should be made, with great care to maintain linearity throughout the range of exposures between densities of 0.8 and 1.8 and also having a sharp toe for "clearing of highlight details". This linear exposure scale produces an instantaneous contrast vs. log exposure curve where the instantaneous contrast reaches a preferred value and does not, in general, exceed that value over the course of the entire exposure range. FIG. 3 of Kuwashima shows the instantaneous contrast having a generally constant value of approximately 1.2 over the entire exposure range; for example, this constant value is due to the linear nature of the D vs. log-E curve. Indeed, they cannot achieve their stated goal of making an exact, faithful reproduction, since they illustrate the preferred reproduction having an instantaneous contrast of 1.2 and not 1.0. To make an exact reproduction would require the contrast of the reproduction to be 1.0 otherwise it would not be an exact reproduction.
Kubotera et al in EP 0 533 033 A1 describe a method for manufacturing a silver halide emulsion in which chemical sensitization is performed at higher temperatures than spectral sensitization. This process leads to a performance improvement in which upper scale contrast is raised. However, they point out that they desire to improve gradation (contrast) while maintaining linearity across the entire exposure range from low to medium to high densities.
Hahm in U.S. Pat. No. 4,902,609 describes a method to improve exposure latitude in a color negative paper by false sensitizing the red light sensitive emulsion with a small amount of green sensitizing dye. The result of this is to desaturate colors such as red by adding a small amount of cyan dye in areas of high exposure. False sensitizing the red sensitive layer with a green sensitizing dye causes cyan dye to be added to colors such as red or blue. The added cyan color darkens the red or blue dye image making it appear to have more contrast.
Similar to Hahm is Waki et al in U.S. Pat. No. 5,084,374 who describe the improvement in shadow detail in high density regions of a color print which is achieved by false sensitizing the green and red light sensitive emulsions with blue sensitizing dye. This technical approach has the same deficiencies as Hahm in desaturating colors.